Cell

ABSTRACT

An electrolytic cell comprising an anode in an anode region and a cathode in a cathode region, the anode region and the cathode regions separated by an ion selective polymer electrolyte membrane; an anolyte in flowing fluid communication with the anode, the anolyte comprising water and a redox mediator couple which is at least partially oxidised at the anode in operation of the cell and at least partially reduced by reaction with water after such oxidation at the anode.

The present invention relates to an electrolysis cell, to components of that cell and for compositions and reagents for use in such cells.

The most commonly used techniques for the bulk production of hydrogen gas use fossil fuels as feedstocks, for example reforming processes. In recent years, there has been an increasing awareness of the environmental cost of obtaining and using fossil fuels and thus industrial processes utilising alternative raw materials are desirable.

A further drawback of producing hydrogen gas from hydrocarbon feedstocks is that the hydrogen gas which is obtained is not suitable for use in applications where hydrogen gas of high purity is required.

Hydrogen has of higher purity may be prepared by electrolysing water. The technique of electrolysing water, i.e. the use of an electrical current to separate water into its constituent elements, hydrogen and oxygen, has been known for several hundred years.

In its simplest form, electrolysis of water is achieved by inserting an anode and a cathode connected to a power source into water. Oxygen (O₂) gas is evolved at the anode and hydrogen (H₂) gas is evolved at the cathode. There are many factors at play which influence the volumes of oxygen and hydrogen gas that are evolved.

To increase the volumes of hydrogen gas that are produced, electrolytes may be added to the water. It is preferable to select an electrolyte with a cation having a lower electrode potential than H⁺, and thus alkali electrolytes comprising cations such as Li⁺, Na⁺, K⁺ and Cs⁺ and are generally preferred.

To further enhance the production of hydrogen gas, electrocatalysts may be employed. One system for performing electrolysis in which such catalysts are used is a polymer electrolyte membrane system. In such systems, a semipermeable membrane separates one or more anodes and cathodes and enables the passage of protons therethrough, but not hydrogen or oxygen gas. The anodes and cathodes may comprise electrocatalysts, most usually noble metal electrocatalysts, or have them applied thereto. Additionally or alternatively, the electrocatalysts may be applied to the polymer electrolyte membrane itself.

While the use of electrocatalysts imparts a marked improvement on the purity of hydrogen gas obtained, those catalysts are generally expensive.

Conventional polymer electrolyte membrane systems are complex as the electrode/membrane/electrode assembly must be formed to maximise the rate of numerous operations, for example delivery of water to the catalyst surface, removal of oxygen, removal of protons to the anode and removal of electrodes to the external circuit.

The size of electrolysis cells is also currently limited and this is in part due to the need to remove the gaseous products from the cell during operation.

It would be desirable to improve the efficiency of generating hydrogen gas via the electrolysis of water such that the cost of doing so would be comparable to or lower than the cost of producing hydrogen gas by more environmentally damaging processes, such as hydrocarbon reforming. This could be achieved by one or more of increasing yields of hydrogen gas, increasing the purity of hydrogen gas, reducing or eliminating the need to use catalysts formed from precious or semi-precious metals.

Thus, according to a first aspect of the present invention, there is provided an electrolytic cell comprising an anode in an anode region and a cathode in a cathode region, the anode region and the cathode regions separated by an ion selective polymer electrolyte membrane; an anolyte in flowing fluid communication with the anode, the anolyte comprising water and a redox mediator couple which is at least partially oxidised at the anode in operation of the cell and at least partially reduced by reaction with water after such oxidation at the anode, the reaction being driven forward by a catalyst comprised in the anode region.

The redox mediator couple (RMC) is at least partially oxidised at the anode according to the following equation:

RMC_(red)→RMC_(ox)+e⁻

The rate of this reaction, expressed in terms of current density, is at least 0.5 A/cm², more preferably at least about 1 A/cm² and most preferably about 1 to 2 A/cm².

Upon contact with water, the oxidised redox mediator couple is at least partially reduced, yielding oxygen gas and protons according to the following equation (hereinafter referred to as the evolution reaction):

RMC_(ox)+2H₂O→RMC_(red)+O₂+4H⁺

This reaction does not occur exclusively at the anode. By enabling the evolution reaction to take place remotely from the anode, a simpler and less costly anode assembly can be employed. It is even envisaged that the evolution of gas could take place completely remotely from the main cell assembly.

The protons formed by the evolution reaction are transferred to the cathode via the polymer electrolyte membrane, resulting in the formation of hydrogen gas.

A further advantage of the present invention is that the amount of precious or semi-precious metal catalyst required to effect the evolution of hydrogen, compared to conventional electrolysis reactions, is either substantially reduced or even eliminated.

A catalyst is employed in the anode region of the cell to drive the evolution reaction forward. The catalyst may be dissolved or suspended in the anolyte liquid. Additionally or alternatively, catalyst may be disposed at one or more fixed locations along the anolyte flow path, for example in the form of a fixed bed type arrangement.

Examples of especially preferred catalysts that may be employed in the anode region of the fuel cell of the present invention include transition metals, especially those residing in Groups 6 to 9, such as manganese, osmium, rhodium, ruthenium, tungsten and/or iridium. Polyoxometallate catalysts are particularly preferable. Especially preferred catalysts are iridium doped ruthenium oxide, [Ru^(III) ₂Zn₂(H₂O)₂(ZnW₉O₃₄)₂]¹⁴⁻, Cs₁₀[Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂], Na₁₄[Ru₂Zn₂(H₂O)₂(ZnW₉O₃₄)₂] and/or Li₁₀[Ru₄(μ-O)₄(μ-OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂].

Other preferred catalysts include those described Yagi et al, Photochem. Photobiol. Sci., 2009, 8, 139-147; in particular di-μ-oxo dimanganese complexes such as [(terpy)(H₂O)Mn(μ-O)Mn(terpy)(H₂O)]³⁺, [Mn₂(mcbpen)₂(H₂O)₂]²⁺, Mn₄O₄ cubane complexes, Mn porphyrin dimers; dinuclear ruthenium complexes such as [(bpy)₂(H₂O)Ru(μ-O)Ru(H₂O)(bpy)₂]⁴⁺, [(terpy)₂(H₂O)Ru(bpp)Ru(H₂O)(terpy)₂]³⁺, [(tBu₂qui)(OH)Ru(btpyan)Ru(OH)(tBu₂qui)]²⁺, and [Ru₂(macroN₆)(Rpy)₄Cl]³⁺; mononuclear ruthenium complexes such as [Ru(tBudnpp)(Rpy)₂OH₂]²⁺ complexes, [Ru(Rterpy)(bpy)OH₂]²⁺ complexes; and iridium complexes, including cyclometalated iridium aquo complexes such as ([Ir^(III)(R₁R₂ppy)₂(OH₂)₂]⁺), wherein R₁ is hydrogen or alkyl, particularly methyl, and wherein R₂ is hydrogen, phenyl or a halogen such as F or Cl.

One factor that may affect the choice of catalyst which is to be used is the identity of the redox mediator couple which is to be used in the anolyte. The redox potential of the redox mediator couple should ideally be greater than 1.23V (the potential of oxygen reduction). However, redox mediator couples exhibiting a significantly higher redox potential than 1.23V are generally undesirable as they reduce the efficiency of the reaction. Redox mediator couples having a redox potential within the range of about 1.25 to about 2.0V are preferred. In especially preferred arrangements, the redox mediator couple exhibits a redox potential of about 1.3V to about 1.8V, about 1.3V to about 1.7V or most preferably from about 1.4V to about 1.6V.

Redox mediator couples which may be employed in the anolytes present in the electrolytic cells of the present invention preferably include lanthanide metal atoms, most preferably cerium^(3+/4+), which has a redox potential (Nernst) of 1.4V. Additional materials, such as acids, may be used to improve the solubility of the redox mediator couple. An examples of an acid which is capable of improving the solubility of cerium is methane sulphonic acid.

The catalyst preferably has a redox potential which is greater than that of oxygen and is ideally similar or up to 200 mV lower, more preferably about 50 to 100 mV lower than that of the mediator.

The anolyte is preferably flowed through the anode region in an anolyte channel, which is preferably cyclical.

As the evolution reaction proceeds, the amount of oxygen gas present in the anolyte stream increases.

To prevent an excessive increase of pressure in the anode region of the electrolytic cell, means are preferably provided to vent the oxygen from the anode region. Thus, in a preferred embodiment, the anolyte is flowed through a separation zone in which at least a portion of the oxygen gas formed by the evolution reaction is separated from the anolyte.

The separation of oxygen gas from the anolyte may be achieved in a number of ways. For example, the evolution of oxygen gas into the anolyte may result in the formation of a foam. The foamed mixture may be flowed into a separation chamber. The flow rate in the separation chamber is preferably lower than the flow rate of the anolyte throughout the remainder of the anolyte region. Passage of the foamed anolyte into the separation chamber results by the natural collapse of the bubbles in the foam. To increase the rate of collapse, cavitation means may be provided. The cavitation means may comprise a cyclone separator which effects the rapid separation of the gas and liquid phases.

The separation zone preferably comprises one or more of: a separation chamber in which the separation of anolyte and oxygen gas takes place; a first inlet port for receiving anolyte and oxygen gas; a first outlet port for supplying anolyte to the anode region of the cell; a second inlet port for receiving a supply of water and/or redox mediator couple and/or catalyst; and a second outlet port for venting oxygen from the chamber.

To reduce, and possibly eliminate, any loss in anolyte solution, one or more demisters may be provided upstream, in or downstream of the second outlet port.

Additionally, to prevent excessive evaporation of water from the anolyte, condensers may be provided upstream, in or downstream of the second outlet port. If a condenser is employed in the electrolytic cell of the present invention, it is preferably arranged such that a predetermined amount of condensate will be returned to the system. Prior to being passed back into the anolyte, the condensate is preferably passed through the demister/s.

In certain arrangements of the present invention, the anolyte channel preferably increases in cross section area in the direction of anolyte flow. This is to accommodate the increasing volume of anolyte and oxygen gas, thus preventing an acceleration in flow rate. The increase in cross section area could be achieved by the channel having a diverging taper. The use of a channel section of increased cross section is especially useful if the formation of oxygen gas predictably occurs at the same point in the anolyte channel, for example if the flowing anolyte is exposed to a packed bed of catalyst (in fixed position) or to catalyst provided on the anode.

At any convenient location in the anolyte channel one or more pumps may be provided to drive circulation of the anolyte solution. Preferably, at least one pump is situated between the downstream end of the separation zone and the upstream end of the anode.

Those skilled in the art will be familiar with the materials which may be used to prepare polymer electrolyte membranes.

The polymer electrolyte membrane is preferably adapted to allow protons to pass through the membrane from the anode side to the cathode side thereof in operation of the cell. The membrane is preferably selective in favour of protons versus other cations.

The membrane may be formed from any suitable material, but preferably comprises a polymeric substrate having cation exchange capability. Suitable examples include fluororesin-type ion exchange resins and non-fluororesin-type ion exchange resins. Fluororesin-type ion exchange resins include perfluorocarboxylic acid resins, perfluorosulfonic acid resins, and the like. Perfluorocarboxylic acid resins are preferred, for example “Nafion”® (Du Pont Inc.), “Flemion”® (Asahi Gas Ltd), “Aciplex” (Asahi Kasei Inc), and the like. Non-fluororesin-type ion exchange resins include polyvinyl alcohols, polyalkylene oxides, styrene-divinylbenzene ion exchange resins, and the like, and metal salts thereof. Preferred non-fluororesin-type ion exchange resins include polyalkylene oxide-alkali metal salt complexes. These are obtainable by polymerizing an ethylene oxide oligomer in the presence of lithium chlorate or another alkali metal salt, for example. Other examples include phenolsulphonic acid, polystyrene sulphonic, polytriflurostyrene sulphonic, sulphonated trifluorostyrene, sulphonated copolymers based on α,β,β triflurostyrene monomer, radiation-grafted membranes. Non-fluorinated membranes include sulphonated poly(phenylquinoxalines), poly(2,6 diphenyl-4-phenylene oxide), poly(arylether sulphone), poly(2,6-diphenylenol); acid-doped polybenzimidazole, sulphonated polyimides; styrene/ethylene-butadiene/styrene triblock copolymers; partially sulphonated polyarylene ether sulphone; partially sulphonated polyether ether ketone (PEEK); polybenzyl suphonic acid siloxane (PBSS).

The electrodes of the cell and the polymer electrolyte membrane are preferably arranged in the cell in a sandwich type construction, the cell comprising an anode chamber on the anode side of the sandwich construction and a cathode chamber on the cathode side of the sandwich construction.

A range of materials that may be used to produce anodes are known to those skilled in the art. The anode in the electrolytic cell of the invention may comprise carbon or metallic materials such as platinum, nickel and/or metal oxide species. However, it is preferable that expensive anodic materials are avoided, and therefore preferred anodic materials include carbon, nickel, metal oxide. The anodic material may be constructed from a fine dispersion of particulate anodic material, the particulate dispersion being held together by a suitable adhesive. The anode may be porous, partially porous or non-porous. The anode is designed to create maximum flow of anolyte to the anode surface. Thus it may consist of shaped flow regulators or a three dimensional electrode; the liquid flow may be managed in a flow-by arrangement where there is a liquid channel adjacent to the electrode, or in the case of the three dimensional electrode, where the liquid is forced to flow through the electrode. The surface of the electrode may also be formed of electrocatalyst, or it may be beneficial to adhere the electrocatalyst in the form of deposited particles on the surface of the electrode.

The anode may be a composite electrode comprise principally anodic material of the type discussed above additionally reinforced with particles of materials such as zirconia, kaolin or zeolite.

In a preferred embodiment of the present invention, the anode takes the form of an anode assembly comprising an anolyte inlet channel and one or more flow channels in fluid communication with the anolyte inlet channel, the flow channels being defined by flow channel walls comprising one or more anode regions, at least one of the flow channels being non-aligned with the anolyte inlet channel.

By “anolyte inlet channel” is meant a channel which carries anolyte into the anode and not merely directs anolyte into the assembly. Thus, an inlet port provided on the outside of an anode assembly chamber wall, directing anolyte into the assembly, would not itself be considered to be an anolyte inlet channel. However, a channel fed by such a port, even if not aligned with the inlet port, which carries anolyte into the assembly, would be considered as an anolyte inlet channel.

In operation of the cell, the anolyte is provided flowing in fluid communication with the anode through the anode assembly of the cell. The redox mediator couple is at least partially oxidised at the anode in operation of the cell, and at least partially reduced by reaction with water.

In a preferred embodiment, the anode includes one or more regions that are porous. In such embodiments, the cell will be arranged such that the anolyte is passed through the porous region/s of the anode, resulting in the redox mediator couple being at least partially oxidised. In arrangements where porous anode regions are employed, the flow channels which they partly define may be closed at one end, so as to force the anolyte flowing into the flow channel through the path of least resistance, i.e. through the porous region/s of the anode. Where the flow channel is closed, the porous anode region is provided at least at the closed end of the flow channel, but may additionally extend along the entirety of the wall defining the flow channel.

Regardless of the porosity of the anode regions, in certain embodiments, substantially all, if not all, of the flow channel walls are formed of an anodic material which may be porous or not.

The use of one or more flow channels which are non-aligned with the anolyte inlet channel effectively creates multiple short flow paths for the anolyte through the anode. A loss of velocity of the anolyte is observed as the anolyte passes through the non-linear flow path. Advantageously, this loss of anolyte velocity reduces any drop in fluid pressure which would otherwise be observed when anolyte is passed through anode assemblies. By minimising the pressure drop, the interaction between the anolyte and the anode regions is maximised. Further, as the loss of flow velocity is offset by the reduction in flow distance, a high overall flow rate of anolyte through the cell can be maintained.

Minimising the pressure drop of anolyte flowing through an anode assembly is especially advantageous in assemblies adapted to direct the flow of anolyte through porous anodes. If there is any pressure drop in the flow of anolyte, the flow resistance of the anolyte through the anode/s will be increased. This will have the effect of reducing the overall flow rate of anolye through the anode assembly. As the viability of electrolytic cells relies on a high flow rate of anolyte therethrough, the rate of anolyte flow through porous anodes, and thus the pressure of that flow, is critical.

Linear flow channels can be effectively employed in the anode assemblies which may be employed in the electrolytic cells of the present invention. However, in alternative embodiments, the flow channels are non-linear and include one or more corners and/or angles.

In most preferred embodiments, the highest possible proportion of flow channels are non-aligned with the anolyte inlet channel.

The anode is preferably designed to create maximum flow of anolyte solution to the anode surface. Thus it may consist of shaped flow regulators or a three dimensional electrode; the liquid flow may be managed in a flow-by arrangement where there is a liquid channel adjacent to the electrode, or in the case of the three dimensional electrode, where the liquid is forced to flow through the electrode.

It is intended that the surface of the electrode is also the electrocatalyst, but it may be beneficial to adhere the electrocatalyst in the form of deposited particles on the surface of the electrode.

In one embodiment, the one or more flow channels extend from the anolyte inlet channel. It will be appreciated that one or more of the non-aligned flow channels will extend from the anolyte inlet channel at an angle. It is this angle which delineates the anolyte inlet channel and the flow channel/s. In preferred embodiments, the one or more flow channels extend from the anolyte inlet channel at an angle of 135 or less, 120° or less, or most preferably 90° or less. For the avoidance of doubt, the angle of projection of the one or more flow channels is measured from the longitudinal axis of the anolyte inlet tube immediately prior (i.e. upstream) of the flow channel.

In an alternative arrangement, the anolyte inlet channel may terminate in an anolyte deposit zone, rather than having flow channels extending therefrom. In such an arrangement, the one or more flow channels may extend from the anolyte deposit zone.

It is preferred that the flow channels be generally parallel. In especially preferred embodiments of the present invention, a plurality of parallel flow channels extend perpendicularly from the anolyte inlet channel.

For the avoidance of any doubt, a flow channel is not considered to be aligned with the anolyte inlet channel merely if it is parallel to that channel.

The anode assembly preferably comprises an anolyte collection zone. The anolyte collection zone is at least partially defined by the outer walls of the one or more flow channels. The walls defining the anolyte collection zone may comprise anode region/s. The anolyte will gather in the anolyte collection zone after it has exited the one or more flow channels.

The anolyte collection zone may comprise collection channels which are preferably formed by the outer walls of the flow channels. In a preferred embodiment, collection channels will be provided between a plurality of parallel flow channels, to provide an interdigitated structure. Anode regions may be provided in the walls defining the collection channels.

The anode assembly is preferably housed in a chamber. The chamber walls may partly define one or more of the anolyte inlet channel, the one or more flow channels, the one or more collection channels and the anolyte collection zone.

The anode assembly is preferably provided with an anolyte outlet channel. In arrangements in which an anolyte collection zone is present, the anolyte outlet channel is preferably in fluid communication with that collection zone. In arrangements in which no anolyte collection zone is present, the anolyte outlet channel is preferably in fluid communication with the at least one flow channels.

According to a second aspect of the present invention, there is provided a method of operating an electrolytic cell comprising:

-   -   a) providing an anode in an anode region and a cathode in a         cathode region of the electrolytic cell, the anode region and         the cathode regions separated by an ion selective polymer         electrolyte membrane;     -   b) providing an anolyte comprising water and a redox mediator         couple     -   c) contacting the anolyte with the anode, causing the anolyte to         be at least partially oxidised at the anode and at least         partially reduced by reaction with water after such oxidation at         the anode, the reaction being driven forward by a catalyst         comprised in the anode region.

For the avoidance of any doubt, where reference has been made above to various aspects of the first aspect of the present invention, specifically, properties of the electrolytic cell and its components and reagents, those properties are equally application to the components and reagents of the second aspect of the present invention.

The invention will now be more particularly described with reference to the following examples.

EXAMPLE 1

FIG. 1 illustrates the anode region of an electrolytic cell (10) of the present invention. The cell (10) comprises a series of cathodes (12) and anodes (14) which are separated by membranes (16). Anolyte is flowed through cyclical anolyte channel (20) around the anolyte region. In the illustrated embodiment, the anolyte comprises a catalyst dissolved therein. This causes the evolution reaction to proceed constantly, regardless of the position of the anolyte within the anolyte region. It will be appreciated that in alternative arrangements, where fixed beds of catalyst are located in or adjacent to the anolyte channel the rate of the evolution reaction will vary depending on the proximity of the anolyte to the catalyst bed.

As the evolution reaction proceeds, protons are produced and these are carried in the anolyte channel to the anode where they pass through the membrane to the cathode where hydrogen gas is formed. This is carried off via line 18.

In addition to the production of protons, oxygen gas is also produced by the evolution reaction. This is carried in the form of a liquid having bubbles 22 entrained therein into separation zone 24.

Separation zone 24 comprises a separation chamber in which the anolyte is passed. The reduced anolyte flow rate in the separation chamber precipitates the cavitation of oxygen bubbles within the anolyte. To accelerate this process, cavitation means in the form of a cyclone separator (not shown) are provided.

The separation zone is provided with an oxygen outlet 26 for venting oxygen from the separation chamber. Demisters and/or condensers may be provided in or downstream of oxygen outlet 26 which return any captured water back to the anolyte. In the event of any loss of water from the anolyte system, additional water can be provided via line 28. 

1. An electrolytic cell comprising an anode in an anode region and a cathode in a cathode region, the anode region and the cathode regions separated by an ion selective polymer electrolyte membrane; an anolyte in flowing fluid communication with the anode, the anolyte comprising water and a redox mediator couple which is at least partially oxidised at the anode in operation of the cell and at least partially reduced by reaction with water after such oxidation at the anode, the reaction being driven forward by a catalyst comprised in the anode region.
 2. The electrolytic cell of claim 1, wherein the catalyst is provided on the surface of the anode and/or the membrane.
 3. The electrolytic cell of claim 1, wherein the catalyst is provided in a fixed bed in the anolyte channel.
 4. The electrolytic cell of claim 1, wherein the catalyst is dissolved or suspended in the anolyte.
 5. The electrolytic cell of claim 1, wherein the catalyst comprises one or more atoms of a group 6 to 9 transition metal.
 6. The electrolytic cell of claim 5, wherein the group 6 to 9 transition metal is manganese, osmium, rhodium, ruthenium, tungsten, and/or iridium.
 7. The electrolytic cell of claim 1, wherein the redox mediator couple has a redox potential greater than 1.25V.
 8. The electrolytic cell of claim 1, wherein the redox mediator couple has a redox potential of 1.3V to 1.8V.
 9. The electrolytic cell of claim 1, wherein the redox mediator couple comprises cerium.
 10. The electrolytic cell of claim 1, wherein the anode region is provided with a separation zone to remove at least some oxygen gas from the anolyte.
 11. The electrolytic cell of claim 10, wherein the separation zone comprises cavitation means to effect the separation of the gas and liquid phases.
 12. The electrolytic cell of claim 10, wherein the separation zone further comprises an oxygen gas outlet.
 13. The electrolytic cell of claim 12, further comprising condensers and/or demisters may be provided upstream, in or downstream of the oxygen gas outlet.
 14. The electrolytic cell of claim 1, wherein the anode is non-porous, partially porous or porous.
 15. The electrolytic cell of claim 1, wherein the anode is a composite electrode.
 16. A method of operating an electrolytic cell comprising: a) providing an anode in an anode region and a cathode in a cathode region of the electrolytic cell, the anode region and the cathode regions separated by an ion selective polymer electrolyte membrane; b) providing an anolyte comprising water and a redox mediator couple c) contacting the anolyte with the anode, causing the anolyte to be at least partially oxidised at the anode and at least partially reduced by reaction with water after such oxidation at the anode, the reaction being driven forwardly by a catalyst comprised in the anode region. 